This is a regression of #244.

With 6.1.0: https://travis-ci.org/twbs/bootstrap/jobs/475278409#L666-L695
With 6.0.1: https://travis-ci.org/twbs/bootstrap/jobs/475230811

It seems to me there must be a check before console.warn.